Pulp

Create a common migration that is used by plugin migrations to migrate content units to the new 2.8 standard storage path.

This approach ensures the migration algorithm is:
- Consistent
- Idempotent (can be resumed)
- Tested
- Memory efficient
- Good performance

The plugin needs to define a migration because it has knowledge of the model.

The plugin knows:
- The collection name.
- The unit keys (needed to calculate the path)
- Whether the storage path is to a directory or individual file.

So, each plugin will define a migration for its content types but all of the heavy lifting is done in the common migration supplied by the platform.